Scientists have found an unusual way to prevent our planet overheating: move it to a cooler spot.

All you have to do is hurtle a few comets at Earth, and its orbit will be altered. Our world will then be sent spinning into a safer, colder part of the solar system.

This startling idea of improving our interplanetary neighbourhood is the brainchild of a group of Nasa engineers and American astronomers who say their plan could add another six billion years to the useful lifetime of our planet - effectively doubling its working life.

'The technology is not at all far-fetched,' said Dr Greg Laughlin, of the Nasa Ames Research Center in California. 'It involves the same techniques that people now suggest could be used to deflect asteroids or comets heading towards Earth. We don't need raw power to move Earth, we just require delicacy of planning and manoeuvring.'

The plan put forward by Dr Laughlin, and his colleagues Don Korycansky and Fred Adams, involves carefully directing a comet or asteroid so that it sweeps close past our planet and transfers some of its gravitational energy to Earth.

'Earth's orbital speed would increase as a result and we would move to a higher orbit away from the Sun,' Laughlin said.

Engineers would then direct their comet so that it passed close to Jupiter or Saturn, where the reverse process would occur. It would pick up energy from one of these giant planets. Later its orbit would bring it back to Earth, and the process would be repeated.

In the short term, the plan provides an ideal solution to global warming, although the team was actually concerned with a more drastic danger. The sun is destined to heat up in about a billion years and so 'seriously compromise' our biosphere - by frying us.

Hence the group's decision to try to save Earth. 'All you have to do is strap a chemical rocket to an asteroid or comet and fire it at just the right time,' added Laughlin. 'It is basic rocket science.'

The plan has one or two worrying aspects, however. For a start, space engineers would have to be very careful about how they directed their asteroid or comet towards Earth. The slightest miscalculation in orbit could fire it straight at Earth - with devastating consequences.

It is a point acknowledged by the group. 'The collision of a 100-kilometre diameter object with the Earth at cosmic velocity would sterilise the biosphere most effectively, at least to the level of bacteria,' they state in a paper in Astrophysics and Space Science. 'The danger cannot be overemphasised.'

There is also the vexed question of the Moon. As the current issue of Scientific American points out, if Earth was pushed out of its current position it is 'most likely the Moon would be stripped away from Earth,' it states, radically upsetting out planet's climate.

These criticisms are accepted by the scientists. 'Our investigation has shown just how delicately Earth is poised within the solar system,' Laughlin admitted. 'Nevertheless, our work has practical implications. Our calculations show that to get Earth to a safer, distant orbit, it would have to pass through unstable zones and would need careful nurturing and nudging. Any alien astronomers observing our solar system would know that something odd had occurred, and would realise an intelligent lifeform was responsible.

'And the same goes for us. When we look at other solar systems, and detect planets around other suns - which we are now beginning to do - we may see that planet-moving has occurred. It will give us our first evidence of the handiwork of extraterrestrial beings.'